Diglycolamide derivatives for separation and recovery of rare earth elements from aqueous solutions

ABSTRACT

Rare earth extractant compounds having the following structure:wherein R1, R2, R3, and R4 are independently selected from alkyl groups containing 1-30 carbon atoms and optionally containing an ether or thioether linkage connecting between carbon atoms, provided that the total carbon atoms in R1, R2, R3, and R4 is at least 12; R5 and R6 are independently selected from hydrogen atom and alkyl groups containing 1-3 carbon atoms; and provided that at least one of the conditions (i)-(iv) apply as follows: presence of a distal branched group in at least one of R1-R4 (condition i), asymmetry in R1-R4 (condition ii), presence of amine-containing ring (condition iii), or presence of lactam ring (condition iv). Also described are hydrophobic water-insoluble solutions containing at least one extractant compound of Formula (1), as well as method for extracting rare earth elements from aqueous solution by contacting the aqueous solution with the water-insoluble solution.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of U.S. Provisional ApplicationNo. 63/048,237, filed on Jul. 6, 2020, all of the contents of which areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention generally relates to rare earth-complexing ligandsand their use in extracting rare earth elements (including lanthanidesand/or actinides) from aqueous solutions into a hydrophobicaqueous-insoluble phase in which the ligand is dissolved. The presentinvention more particularly relates to diglycolamide derivatives as rareearth-extracting agents.

BACKGROUND OF THE INVENTION

Rare earth elements (REEs) generally include 15 lanthanides(lanthanum-lutetium), scandium, and yttrium. REEs possess uniquephysical and chemical properties and are integral for a variety oftechnology applications, ranging from energy generation to defense andmilitary. The Department of Energy (DOE) and European Commission havenamed REEs as “critical raw materials” based on the challengesassociated with their separation and continuous increase in globalconsumption (Sholl, D. S.; Lively, R. P. Seven chemical separations tochange the world, Nature 2016, 533, 316-316; Critical MaterialsStrategy, Report DOE/PI-0009, U.S. Department of Energy, Washington, DC,December 2011). Lack of a domestic supply chain of pure REEs has leftthe United States dependent on other nations. Moreover, the low minableconcentration of REEs in the Earth's crust makes it difficult toeconomically recover and separate them due to their very similarchemical and physical properties.

Solvent (liquid-liquid) extraction is the industry standard used toseparate rare earth elements (REE) from aqueous acidic solutions.Liquid-liquid separation offers a continuous operation and sizableproduction capacity. The organophosphorus compound2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester, also known underthe common names EHEHPA or PC88A, is a well known extractant usedindustrially to separate the light REEs (lanthanum-gadolinium) fromheavy REEs (yttrium, terbium-lutetium) as well as to purify individualelements. However, the adjacent lanthanide selectivity is quite low,with a reported selectivity for Nd over Pr in the REE(III)-HCl-EHEHPAsystem being just 1.17 (Xie, F. et al., Minerals Engineering, 2014, 56,10-28). Another drawback of the EHEHPA process is the chemical natureand pH-swing mechanism by which the organophosphorus acids operate,which requires the consumption of acid and base via saponification toovercome the adverse liberation of acid encountered during REEextraction.

The diglycolamides (DGAs)—tridentate O-based neutral ligands—at firstdeveloped for actinide/lanthanide partitioning in the nuclear fuelcycle, have been investigated as effective alternatives to acidicextractants to achieve efficient separation of heavy REEs from lightREEs (reported average adjacent-lanthanide separation factor for lightREEs La-Pr in the REE(III)-HCl-TODGA system is 2.618). An advantage ofthese neutral ligands is that they do not require saponification, whichresults in less chemicals used and less waste produced. Some extensivelystudied diglycolamides include N,N,N′,N′-Tetra(n-octyl)diglycolamide(TODGA), N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide (TEHDGA), andN,N-dimethyl-N′,N′-di(n-octyl)diglycolamide (DMDODGA).

TEHDGA, having branched alkyl substituents on amide nitrogens, with thebranching point close to the binding site, shows low affinity andselectivity across the lanthanide series, whereas TODGA (which has nobranching) shows strong preference for heavy lanthanides, which resultsin acceptable separation of the light lanthanides, most notably, Nd/Pr.In addition, TODGA has higher separation factors than PC88A, whichreduces stage requirements to facilitate separation. However, asubstantial drawback of current diglycolamides is their tendency to forma third phase characterized by gelling/precipitation at higherdiglycolamide concentrations/loading (above 0.1 M) in the organic phaseand also generally requires consumption of large quantities of acidthroughout the separation stages. There would be an advantage in a newextraction method that could effectively extract rare earth elementsfrom acidic aqueous solutions while avoiding gelling or precipitation inthe organic phase and without requiring consumption of large quantitiesof acid.

Another challenge is that REE sources from industrial byproduct streams(e.g., phosphoric acid production) often contain undesired material suchas uranium and thorium. The economic viability of REE recovery from suchstreams is significantly hampered if material needs to be removed fromthe product stream. This is particularly true for the radioactiveelements thorium and uranium. Moreover, REEs are critical components formany modern technologies, including those of renewable energy. Toincrease the domestic supply of REEs, new and more effective methods forextracting REEs from industrial byproduct streams are needed. Therewould also be an advantage in an extraction method that can remove oneor more REEs more selectively than one or more other REEs, so as topermit a separation of REEs. There would be a further advantage in sucha method using straight-forward and low-cost means for extraction andseparation of REEs.

SUMMARY OF THE INVENTION

In a first aspect, the present disclosure is directed todiglycolamide-based compounds that can advantageously extract rare earthelements from acidic aqueous solutions with minimal or no formation of athird gelling or precipitation phase, and preferably, with sufficient oreven greater separation ability than known extractant compounds. Theextractant compound can preferably achieve this at higher extractantcompound concentrations, e.g., of at least or above 0.1, 0.2, or 0.3 M.The rare earth extractant compounds described herein can alsoadvantageously operate at lower acid concentrations than traditionallyused in the art.

The rare earth extractant compound has the following structure:

In Formula (1), R¹, R², R³, and R⁴ are independently selected from alkylgroups containing 1-30 carbon atoms and optionally containing an etheror thioether linkage connecting between carbon atoms, provided that thetotal carbon atoms in R¹, R², R³, and R⁴ is at least 12; R⁵ and R⁶ areindependently selected from hydrogen atom and alkyl groups containing1-3 carbon atoms; and provided that at least one of the followingconditions apply:

Condition (i): at least one of R¹, R², R³, and R⁴ is a distal branchedalkyl group constructed of a linear alkyl backbone having at least fourcarbon atoms with an alpha carbon atom of the linear alkyl backboneattached to a nitrogen atom shown in Formula (1), and the linear alkylbackbone contains a substituting hydrocarbon group at a gamma carbon orhigher positioned carbon on the linear alkyl backbone, wherein thesubstituting hydrocarbon group contains at least one carbon atom,provided that the distal branched alkyl group contains a total of up to30 carbon atoms;

Condition (ii): R¹ and R² are equivalent and R³ and R⁴ are separatelyequivalent, while R¹ and R² are different from R³ and R⁴, to result inan asymmetrical compound of Formula (1); or alternatively, only one ofR¹, R², R³, and R⁴ is different, to result in an asymmetrical compoundof Formula (1).

Condition (iii): R¹ and R² interconnect to form a first amine-containingring, and/or R³ and R⁴ interconnect to form a second amine-containingring, and the first and second amine-containing rings are attached to analkyl group containing at least three carbon atoms and optionallycontaining an ether or thioether linkage connecting between carbonatoms, wherein the total number of carbon atoms in the first or secondamine-containing ring and attached alkyl group is up to 30 carbon atoms;and/or

Condition (iv): R² and R⁵ interconnect to form a first lactam ring,and/or R⁴ and R⁶ interconnect to form a second lactam ring.

In a second aspect, the present disclosure is directed to a liquidsolution useful for extracting rare earth elements (REEs) from anaqueous solution, typically an acidified aqueous solution. The liquidsolution contains at least one rare earth extractant compound describedabove dissolved in an aqueous-insoluble (hydrophobic) solvent. Theaqueous-insoluble hydrophobic solvent may be, for example, a hydrocarbonsolvent. In some embodiments, the liquid solution further contains anorganoamine soluble in the aqueous-insoluble hydrophobic solvent. Theorganoamine may contain at least one hydrocarbon group containing atleast four carbon atoms. In other embodiments, the liquid solutionfurther contains an alcohol soluble in the aqueous-insoluble hydrophobicsolvent. The alcohol may contain at least six, seven, or eight carbonatoms.

In a third aspect, the present disclosure is directed to a method forextracting one or more rare earth elements from an aqueous solution. Themethod includes the following steps, at minimum: (i) acidifying anaqueous solution containing the one or more rare earth elements with anorganic acid to result in an acidified aqueous solution containing theone or more rare earth elements and containing the inorganic acid in aconcentration of 1-12 M, wherein the rare earth elements are selectedfrom lanthanides, actinides, or combination thereof, and (ii) contactingthe acidified aqueous solution with the aqueous-insoluble (hydrophobic)extractant solution described above to result in extraction of one ormore of the rare earth elements into the extractant solution by bindingof the rare earth extractant compound to the one or more rare earthelements. In some embodiments, the method further includes: (iii)stripping one or more rare earth elements from the extractant solutionby contacting the extractant solution with an aqueous stripping solutionof an inorganic acid wherein the inorganic acid is present in theaqueous stripping solution in a concentration of no more than 4 M, andprovided that the concentration of inorganic acid in the aqueousstripping solution is at least 0.5 M less than the concentration ofinorganic acid in the aqueous solution in step (i).

Notably, the extraction process described herein is advantageouslystraight-forward and cost-efficient while at the same time capable ofremoving a substantial portion or all of the REEs from an aqueoussource, and further capable of separating REEs from each other by eitherselective extraction, selective stripping, or both. The extractionprocess can also advantageously extract rare earth elements from acidicaqueous solutions with minimal or no formation of a third gelling orprecipitation phase, even when operated at higher extractant compoundconcentrations, such as at least 0.1 M, 0.2 M, 0.5 M, or 1 M. The abovedescribed extraction process can also advantageously operate at loweracid concentrations than traditionally used in the art, e.g., at 1-8 Mor 2-6 M.

In some embodiments, the extraction solution exhibits a degree ofselectivity in extracting the REEs, i.e., by extracting one or more REEsto a greater extent than one or more other REEs. In some embodiments,the extraction solution further includes an organoamine compound solublein the aqueous-insoluble hydrophobic solvent, wherein the organoaminepreferably contains at least one hydrocarbon group containing at leastfour carbon atoms. In alternative or further embodiments, the extractantsolution further includes an alcohol soluble in the aqueous-insolublehydrophobic solvent, wherein the alcohol preferably contains at leastsix carbon atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting variation of log D in the extraction ofLn(III), 0.5 mM each, with 0.1 M DGA ligands 1 (TODGA), 2 (TEHDGA), and3 (DMDODGA) from 3 M HCl media into Isopar L with 30 vol % of Exxal™ 13at 25° C. after 1 hour. The dotted horizontal line represents theleveling off point.

FIGS. 2A-2H are plots showing variation of log D in the extraction ofLn(III), 0.5 mM each, with 0.1 M DGA ligands 1-21 from 3 M HCl mediainto Isopar L with 30 vol % of Exxal 13 at 25° C. after 1 hour. The logD value for Eu(III) using ligand 3 is missing due to completeextraction, i.e., Eu³⁺ concentration is below the detection limit byICP-OES. The same is true for Dy(III), Er(III)-Lu(III) using ligand 9.

FIGS. 3A-3C are plots showing change in log D for Pr(III) when: a)varying size of two N,N′-linear alkyl substituents in N,N′-di(n-octyl)DGA structure (FIG. 3A); b) varying size of two N,N′-branched alkylsubstituents in N,N′-dimethyl-substituted DGA (FIG. 3B); and c) varyingsize of two N,N′-branched alkyl substituents inN,N′-di(n-octyl)-substituted DGA (FIG. 3C).

FIGS. 4A-4C are plots showing change in log D for Gd(III) when: a)decreasing steric hindrance around the binding site in DGA (FIG. 4A); b)moving branching site on N,N′-substituent further away from binding sitein N,N′-dimethyl-substituted DGA (FIG. 4B); and c) moving branching siteon N,N′-substituent further away from binding site inN,N′-di(n-octyl)-substituted DGA (FIG. 4C).

FIG. 5 is a graph showing Eu extraction capability of ligands 2, 11, and13 from 3M HCl solution into Isopar L with 30 vol % of Exxal 13 at 25°C. Concentration of Eu in the acidic solution is 1.2 mM.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “hydrocarbon group” (also denoted by the groupR) is defined as a chemical group composed solely of carbon andhydrogen, except that the hydrocarbon group may (i.e., optionally) besubstituted with one or more fluorine atoms to result in partial orcomplete fluorination of the hydrocarbon group, or the hydrocarbon groupmay or may not also contain a single ether or thioether linkageconnecting between carbon atoms in the hydrocarbon group. Thehydrocarbon group typically contains 1-30 carbon atoms. In differentembodiments, one or more of the hydrocarbon groups may contain, forexample, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 18, 20, 22, 24, 26, 28,or 30 carbon atoms, or a number of carbon atoms within a particularrange bounded by any two of the foregoing carbon numbers (e.g., 1-30,2-30, 3-30, 4-30, 6-30, 8-30, 10-30, 12-30, 1-20, 6-20, 8-20, 10-20, or12-20 carbon atoms). Hydrocarbon groups in different compounds describedherein, or in different positions of a compound, may possess the same ordifferent number (or preferred range thereof) of carbon atoms in orderto independently adjust or optimize such properties as the complexingability, extracting (extraction affinity) ability, selectivity ability,or third phase prevention ability of the compound.

In a first set of embodiments, the hydrocarbon group (R) is a saturatedand straight-chained group, i.e., a straight-chained (linear) alkylgroup. Some examples of straight-chained alkyl groups include methyl,ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl,n-hexadecyl, n-heptadecyl, n-octadecyl, n-eicosyl, n-docosyl,n-tetracosyl, n-hexacosyl, n-octacosyl, and n-triacontyl groups.

In a second set of embodiments, the hydrocarbon group (R) is saturatedand branched, i.e., a branched alkyl group. Some examples of branchedalkyl groups include isopropyl (2-propyl), isobutyl (2-methylprop-1-yl),sec-butyl (2-butyl), t-butyl (1,1-dimethylethyl-1-yl), 2-pentyl,3-pentyl, 2-methylbut-1-yl, isopentyl (3-methylbut-1-yl),1,2-dimethylprop-1-yl, 1,1-dimethylprop-1-yl, neopentyl(2,2-dimethylprop-1-yl), 2-hexyl, 3-hexyl, 2-methylpent-1-yl,3-methylpent-1-yl, isohexyl (4-methylpent-1-yl), 1,1-dimethylbut-1-yl,1,2-dimethylbut-1-yl, 2,2-dimethylbut-1-yl, 2,3-dimethylbut-1-yl,3,3-dimethylbut-1-yl, 1,1,2-trimethylprop-1-yl,1,2,2-trimethylprop-1-yl, isoheptyl, isooctyl, and the numerous otherbranched alkyl groups having up to 20 or 30 carbon atoms, wherein the“1-yl” suffix represents the point of attachment of the group.

In a third set of embodiments, the hydrocarbon group (R) is saturatedand cyclic, i.e., a cycloalkyl group. Some examples of cycloalkyl groupsinclude cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,and cyclooctyl groups. The cycloalkyl group can also be a polycyclic(e.g., bicyclic) group by either possessing a bond between two ringgroups (e.g., dicyclohexyl) or a shared (i.e., fused) side (e.g.,decalin and norbornane).

In a fourth set of embodiments, the hydrocarbon group (R) is unsaturatedand straight-chained, i.e., a straight-chained (linear) olefinic oralkenyl group. The unsaturation occurs by the presence of one or morecarbon-carbon double bonds and/or one or more carbon-carbon triplebonds. Some examples of straight-chained olefinic groups include vinyl,propen-1-yl (allyl), 3-buten-1-yl (CH₂═CH—CH₂—CH₂—), 2-buten-1-yl(CH₂—CH═CH—CH₂—), butadienyl, 4-penten-1-yl, 3-penten-1-yl,2-penten-1-yl, 2,4-pentadien-1-yl, 5-hexen-1-yl, 4-hexen-1-yl,3-hexen-1-yl, 3,5-hexadien-1-yl, 1,3,5-hexatrien-1-yl, 6-hepten-1-yl,ethynyl, propargyl (2-propynyl), 3-butynyl, and the numerous otherstraight-chained alkenyl or alkynyl groups having up to 20 or 30 carbonatoms.

In a fifth set of embodiments, the hydrocarbon group (R) is unsaturatedand branched, i.e., a branched olefinic or alkenyl group. Some examplesof branched olefinic groups include propen-2-yl (CH₂═C.—CH₃),1-buten-2-yl (CH₂═C.—CH₂—CH₃), 1-buten-3-yl (CH₂═CH—CH.—CH₃),1-propen-2-methyl-3-yl (CH₂═C(CH₃)—CH₂—), 1-penten-4-yl, 1-penten-3-yl,1-penten-2-yl, 2-penten-2-yl, 2-penten-3-yl, 2-penten-4-yl, and1,4-pentadien-3-yl, and the numerous other branched alkenyl groupshaving up to 20 or 30 carbon atoms, wherein the dot in any of theforegoing groups indicates a point of attachment.

In a sixth set of embodiments, the hydrocarbon group (R) is unsaturatedand cyclic, i.e., a cycloalkenyl group. The unsaturated cyclic group maybe aromatic or aliphatic. Some examples of unsaturated cyclichydrocarbon groups include cyclopropenyl, cyclobutenyl, cyclopentenyl,cyclopentadienyl, cyclohexenyl, cyclohexadienyl, phenyl, benzyl,cycloheptenyl, cycloheptadienyl, cyclooctenyl, cyclooctadienyl, andcyclooctatetraenyl groups. The unsaturated cyclic hydrocarbon group mayor may not also be a polycyclic group (such as a bicyclic or tricyclicpolyaromatic group) by either possessing a bond between two of the ringgroups (e.g., biphenyl) or a shared (i.e., fused) side, as innaphthalene, anthracene, phenanthrene, phenalene, or indene fused ringsystems.

As indicated earlier above, any of the hydrocarbon groups describedabove may be substituted with one or more fluorine atoms. As an example,an n-octyl group may be substituted with a single fluorine atom toresult in, for example, a 7-fluorooctyl or 8-fluorooctyl group, orsubstituted with two or more fluorine atoms to result in, for example,7,8-difluorooctyl, 8,8-difluorooctyl, 8,8,8-trifluorooctyl, orperfluorooctyl group. As also indicated earlier above, any of thehydrocarbon groups described above may contain a single ether (—O—) orthioether (—S—) linkage connecting between carbon atoms in thehydrocarbon group. An example of a hydrocarbon group containing a singleether or thioether group is —(CH₂)₂—X—(CH₂)₇CH₃, wherein X represents Oor S.

In one aspect, the present disclosure is directed to rare earthextractant compounds having an ability to complex with a rare earthelement (i.e., REE) in solution and transfer (extract) the rare earthelement from an aqueous solution into an aqueous-insoluble hydrophobic(non-polar) solution in which the extractant compound is dissolved. Theextractant compound contains a diglycolamide moiety and at least one,two, three, or four hydrocarbon groups that render the diglycolamidemolecule soluble in a non-polar aqueous-insoluble solvent, such as ahydrocarbon solvent. The term “compound” is herein meant to besynonymous with the term “molecule”.

In particular embodiments, the extractant compound has a structurewithin the following generic structure:

In Formula (1) above, R¹, R², R³, and R⁴ are independently selected fromlinear, branched, or cyclic alkyl groups (R′) containing 1-30 carbonatoms, as described above, provided that the total carbon atoms in R¹,R², R³, and R⁴ (i.e., the sum of carbon atoms in all of R¹, R², R³, andR⁴) is at least 12. In different embodiments, the total carbon atoms inR¹, R², R³, and R⁴ is at least 12, 13, 14, 15, 16, 18, 20, 22, 24, 26,28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 60, 64, 68,70, 72, 76, or 80, or a total carbon number within a range bounded byany two of the foregoing values (e.g., 12-80). In some embodiments, R¹,R², R³, and R⁴ are the same, such as in the case where R¹, R², R³, andR⁴ are each isododecyl, i.e., —(CH₂)₉CH(CH₃)₂, in which case the totalcarbon number provided by R¹, R², R³, and R⁴ is 48. The term “same,” asused herein, refers at least to the same carbon number in two or more ofR¹, R², R³, and R⁴, and the term may further refer to the samestructure. In other embodiments, at least one of R¹, R², R³, and R⁴ isdifferent from another of R¹, R², R³, and R⁴, such as in the case whereR¹ and R³ are methyl groups and R² and R⁴ are isooctyl groups (whichresults in a symmetric structure), or where R¹ and R² are methyl groupsand R³ and R⁴ are isooctyl groups (which results in an asymmetricstructure). In either of the foregoing cases, the total carbon numberprovided by R¹, R², R³, and R⁴ is 18. As noted in the above examples,the structure according to Formula (1) may be symmetric or asymmetric.Another example of an asymmetric structure is one in which R¹, R², andR³ are equivalent to each other while different from R⁴.

The groups R⁵ and R⁶ in Formula (1) above are independently selectedfrom hydrogen atom and hydrocarbon groups containing 1-3 carbon atoms.In a first set of embodiments, R⁵ and R⁶ are hydrogen atoms. In a secondset of embodiments, R⁵ and R⁶ are hydrocarbon groups containing 1-3carbon atoms. In a third set of embodiments, one of R⁵ and R⁶ is ahydrogen atom and the other is a hydrocarbon group containing 1-3 carbonatoms. In the case where one or both of R⁵ and R⁶ is a hydrocarbon, thehydrocarbon is typically an alkyl group, typically containing 1-6 carbonatoms, and more particularly, a methyl, ethyl, n-propyl, or isopropylgroup.

In some embodiments, R¹, R², R³, and R⁴ are all alkyl groups, which maybe the same or different. A sub-class of Formula (1) in which R¹, R²,R³, and R⁴ are all alkyl groups can be described by the followingsub-formula:

wherein m, n, p, and q are each independently an integer of 0-20,provided that the sum of m, n, p, and q is at least 8, and where R⁵ andR⁶ are as defined above. In some embodiments, m, n, p, and q are thesame, such as m, n, p, and q all being 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, or 19. In other embodiments, m, n, p, and q arenot all the same, such as m and q being 0 and n and p each being 4, 5,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or 19; or, as anotherexample, m and q being 1 or 2 and n and p each being 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or 19. Moreover, any one or morehydrogen atoms in methylene groups in Formula (1a) may optionally bereplaced with a methyl, ethyl, n-propyl, or isopropyl group, to resultin a branched hydrocarbon group, provided that the branched hydrocarbongroup contains up to 20 carbon atoms, as provided in Formula (1).

Some examples of specific compounds under Formula (1a) in which allalkyl groups corresponding to R¹, R², R³, and R⁴ are the same areprovided as follows:

Some examples of specific compounds under Formula (1a) in which not allalkyl groups corresponding to R¹, R², R³, and R⁴ are the same areprovided as follows:

In some embodiments of Formula (1), a first condition applies in whichat least one (e.g., one, two, three, or all) of R¹, R², R³, and R⁴ is adistal branched alkyl group constructed of a linear alkyl backbonehaving at least four, five, six, seven, eight, nine, ten, eleven, ortwelve carbon atoms with an alpha carbon atom of the linear alkylbackbone attached to a nitrogen atom shown in Formula (1), and thelinear alkyl backbone contains a substituting hydrocarbon group (whichmay be an alkyl group) at a gamma carbon or higher positioned carbon onthe linear alkyl backbone. The substituting hydrocarbon group can be anyof the hydrocarbon groups described above containing at least one or twocarbon atoms, provided that the total number of carbon atoms in thedistal branched alkyl group is up to 30 carbon atoms. In particularembodiments, one or more of the substituting hydrocarbon groups contain1-6 carbon atoms, such as those selected from methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, cyclopentyl,cyclohexyl, and phenyl groups. The linear alkyl backbone may be depictedas follows, with alpha, beta, gamma, delta, and epsilon positionsdenoted:

—(CH₂)_(α)(CH₂)_(β)(CH₂)_(γ)(CH₂)_(δ)(CH₂)_(ε)(CH₂)_(n)(CH₃),

wherein n is 0 or a number of 1 or greater. In some embodiments, thedistal branched alkyl group contains precisely or at least onesubstituting hydrocarbon group located at a gamma carbon, delta carbon,epsilon carbon, or higher carbon position (e.g., zeta, eta, theta, iota,or kappa) of the linear alkyl backbone. In other embodiments, the distalbranched alkyl group contains at least two (or more) substitutinghydrocarbon groups independently located at a gamma carbon, deltacarbon, epsilon carbon, or higher carbon position (e.g., zeta, eta,theta, iota, or kappa) or combination of such positions of the linearalkyl backbone.

Some examples of distal branched alkyl groups according to condition 1include:

The compounds according to Formula (1) and sub-formulas thereof can besynthesized by methods well known in the art. Reference is made to, forexample, D. D. Dicholkar et al., Ind. Eng. Chem. Res., 52(7), 2457-2469,2013, which describes the synthesis ofN,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) in detail. TheExamples, provided later below, describe a number of methods forproducing these compounds.

In some embodiments of Formula (1), a second condition applies in whichR¹ and R² are equivalent and R³ and R⁴ are separately equivalent, whileR¹ and R² are different from R³ and R⁴, to result in an asymmetricalcompound of Formula (1). In some embodiments, R¹ and R² are equivalenthydrocarbon groups (or more particularly, alkyl groups) containing 1-3carbon atoms, and R³ and R⁴ are separately equivalent hydrocarbon groupscontaining 4-30, 6-30, 8-30, 10-30, 12-30, 4-20, 6-20, 8-20, 10-20, or12-20 carbon atoms, wherein all such hydrocarbon groups have beendescribed above. For example, R¹ and R² may both be methyl or ethyl andR³ and R⁴ may both be the same C₃-C₃₀, C₄-C₃₀, C₅-C₃₀, C₆-C₃₀, C₇-C₃₀,or C₈-C₃₀, linear, branched, or cyclic alkyl group, as described above,such as n-propyl, isopropyl, cyclopropyl, n-butyl, sec-butyl, isobutyl,t-butyl, cyclobutyl, n-pentyl, isopentyl, n-hexyl, isohexyl, cyclohexyl,n-octyl, isooctyl, n-decyl, n-undecyl, n-dodecyl, n-tetradecyl,n-hexadecyl, or larger group with or without substitution with one ormore hydrocarbon groups (R) as described above. Alternatively, only oneof R¹, R², R³, and R⁴ is different to result in an asymmetricalcompound. For example, R¹ may be methyl or ethyl and R², R³, and R⁴ mayall be the same C₃-C₃₀, C₄-C₃₀, C₅-C₃₀, C₆-C₃₀, C₇-C₃₀, or C₈-C₃₀,linear, branched, or cyclic alkyl group, as described above, such asn-propyl, isopropyl, cyclopropyl, n-butyl, sec-butyl, isobutyl, t-butyl,cyclobutyl, n-pentyl, isopentyl, n-hexyl, isohexyl, cyclohexyl, n-octyl,isooctyl, n-decyl, n-undecyl, n-dodecyl, n-tetradecyl, n-hexadecyl, orlarger group with or without substitution with one or more hydrocarbongroups (R) as described above. Any of the alkyl groups described abovemay or may not contain fluorine substitution and/or an ether orthioether linkage connecting between carbon atoms.

In some embodiments of Formula (1), a third condition applies in whichR¹ and R² interconnect to form a first amine-containing ring, and/or R³and R⁴ interconnect to form a second amine-containing ring. The firstand second amine-containing rings are typically attached to at least onealkyl group containing at least three, four, five, six, seven, or eightcarbon atoms (and up to 12, 18, 20, 24, or 30 carbon atoms), wherein thealkyl group optionally contains fluorine substitution and optionallycontains an ether or thioether linkage connecting between carbon atoms.The total number of carbon atoms in the first or second amine-containingring and attached alkyl group (combined) is typically up to 30 carbonatoms. The amine-containing ring is typically a five-membered orsix-membered ring. The amine-containing ring may be substituted with anadditional hydrocarbon group (R), such as any of those described above,particularly alkyl groups containing 1-4 carbon atoms, such as a methyl,ethyl, n-propyl, isopropyl group. n-butyl, sec-butyl, isobutyl, ort-butyl group. Some examples of extractant compounds according to thethird condition of Formula (1) include:

In the above structures (1b-1), (1b-2), and (1b-3), R⁷, R⁸, R⁹, R¹⁰,R¹¹, R¹², R¹³, and R¹⁴ are independently selected from hydrogen atom (H)and hydrocarbon groups (R) described earlier above, provided that atleast one of R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ is a hydrocarbongroup (R) containing 1-30 carbon atoms. In some embodiments, at leastone of R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ is an alkyl groupcontaining 1-30 carbon atoms. In some embodiments, at least one of R⁷,R⁸, R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ is a methyl, ethyl, n-propyl, orisopropyl group. In some embodiments, at least one of R⁷, R⁸, R⁹, R¹⁰,R¹¹, R¹², R¹³, and R¹⁴ is a hydrocarbon group (or more particularly, analkyl group) containing 4-30, 6-30, 8-30, 10-30, 12-30, 4-20, 6-20,8-20, 10-20, or 12-20 carbon atoms. In particular embodiments, R⁹ and/orR¹³ is a hydrocarbon group, or more particularly an alkyl group,containing 1-30 carbon atoms or any of the particular sub-ranges ofcarbon atoms provided above.

In some embodiments of Formula (1), a fourth condition applies in whichR² and R⁵ interconnect to form a first lactam ring, and/or R⁴ and R⁶interconnect to form a second lactam ring. The lactam ring is typicallya five-membered or six-membered ring. The lactam ring may also besubstituted with any of the hydrocarbon groups (R) described abovecontaining 1-30 carbon atoms, including methyl, ethyl, n-propyl, andisopropyl groups. Some examples of extractant compounds according to thefourth condition of Formula (1) include:

In the above structures (1c-1), (1c-2), (1c-3), and (1c-4), R¹⁵, R¹⁶,R¹⁷, and R¹⁸ are independently selected from hydrogen atom (H) andhydrocarbon groups (R) described earlier above. In some embodiments, atleast one, two, three, or all of R¹⁵, R¹⁶, R¹⁷, and R¹⁸ are hydrogenatoms. In other embodiments, precisely or at least one of R¹⁵, R¹⁶, R¹⁷,and R¹⁸ is a hydrocarbon group (R) or more particularly a linear,branched, or cyclic alkyl group containing precisely or at least 3, 4,5, 6, 7, 8, 9, 10, 11, or 12 and up to 14, 16, 18, 20, 22, 24, 26, 28,or 30 carbon atoms, or an alkyl group containing a number of carbonatoms within a range bounded by any two of the foregoing values (e.g.,3-30, 4-30, 6-30, 8-30, 10-30, 12-30, 3-20, 4-20, 6-20, 8-20, 10-20, or12-20 carbon atoms). In typical embodiments, at least one (or both) ofR¹ and R³ in the above structures (1c-1), (1c-2), (1c-3), and (1c-4) isa linear, branched, or cyclic alkyl group containing precisely or atleast 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 and up to 14, 16, 18, 20, 22,24, 26, 28, or 30 carbon atoms, or an alkyl group containing a number ofcarbon atoms within a range bounded by any two of the foregoing values(e.g., 3-30, 4-30, 6-30, 8-30, 10-30, 12-30, 3-20, 4-20, 6-20, 8-20,10-20, or 12-20 carbon atoms). In other embodiments, at least one (orall) of R¹, R³, and R⁴ is a linear, branched, or cyclic alkyl groupcontaining precisely or at least 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 andup to 14, 16, 18, 20, 22, 24, 26, 28, or 30 carbon atoms, or an alkylgroup containing a number of carbon atoms within a range bounded by anytwo of the foregoing values (e.g., 3-30, 4-30, 6-30, 8-30, 10-30, 12-30,3-20, 4-20, 6-20, 8-20, 10-20, or 12-20 carbon atoms).

In another aspect, the present disclosure is directed to a liquidextractant solution useful for extracting rare earth elements fromaqueous solutions. The extraction solution includes one or moreextractant compounds (i.e., any one or more of the diglycolamidecompounds of Formula (1) or sub-formulas thereof or species thereof,described above), dissolved in an aqueous-insoluble hydrophobic solvent.The aqueous-insoluble hydrophobic solvent can be any of the hydrophobicorganic solvents known in the art that are substantially or completelyimmiscible with water or aqueous solutions in general. Theaqueous-insoluble hydrophobic solvent is typically a hydrocarbonsolvent, which may be non-halogenated (e.g., hexanes, heptanes, octanes,decanes, dodecanes, benzene, toluene, xylenes, kerosene, or petroleumether), or halogenated (e.g., methylene chloride, chloroform, carbontetrachloride, 1,2-dichlorethane, trichloroethylene, andperchloroethylene), or etherified (e.g., diethyl ether or diisopropylether), or combination of halogenated and etherified (e.g.,bis(chloroethyl)ether and 2-chloroethyl vinyl ether). In someembodiments, the extractant solution is composed solely of theextractant compound and the aqueous-insoluble hydrophobic solvent. Inother embodiments, the extractant solution contains one or moreadditional components, as further discussed below. The one or moreextractant compounds may be present in the extractant solution in aconcentration of, for example, precisely, at least, or up to, forexample, 0.01 M, 0.02 M, 0.05 M, 0.1 M, 0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6M, 0.7 M, 0.8 M, 0.9 M, or 1 M or a concentration within a range boundedby any two of the foregoing values, e.g., 0.01-1 M, 0.01-0.5 M, 0.05-1M, 0.05-0.5 M, 0.1-1 M, 0.1-0.8 M, 0.1-0.5 M, 0.2-1 M, 0.2-0.8 M, or0.2-0.5 M.

In some embodiments, the extractant solution, as described above,further includes an organoamine soluble in the aqueous-insolublehydrophobic solvent. The organoamine may function to, for example,further bind to the REE, prevent or lessen formation of a third phaseduring the extraction, and/or assist in removing (stripping) the REEfrom the aqueous-insoluble hydrophobic solvent after extraction. To besoluble in the hydrophobic solvent, the organoamine should besufficiently hydrophobic (lipophilic). To be sufficiently hydrophobic,the organoamine should contain at least one hydrocarbon group containingat least four carbon atoms. However, to ensure full solubility of theorganoamine in the hydrophobic solvent, the organoamine preferablycontains, in total, at least or more than six carbon atoms. In differentembodiments, the organoamine may contain at least or more than, forexample, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20carbon atoms, or a number of carbon atoms within a range bounded by anytwo of the foregoing values. The organoamine may be a primary,secondary, or tertiary amine. Some examples of primary organoaminesinclude n-hexylamine, isohexylamine, n-heptylamine, n-octylamine,isooctylamine, n-nonylamine, n-decylamine, n-undecylamine,n-dodecylamine, n-tridecylamine, n-tetradecylamine, andn-hexadecylamine. Some examples of secondary organoamines includedibutylamine, diisobutylamine, dipentylamine, dihexylamine,diheptylamine, diooctylamine, dinonylamine, didecylamine,didodecylamine, N-methylbutylamine, N-methylpentylamine,N-methylhexylamine, N-methylheptylamine, N-methyloctylamine,N-ethylbutylamine, and N-ethyloctylamine. Some examples of tertiaryorganoamines include tributylamine, tripentylamine, trihexylamine,triheptylamine, trioctylamine, trinonylamine, tridecylamine,triundecylamine, and tridodecylamine.

In some embodiments, the extractant solution, as described above,further includes an organoamide soluble in the aqueous-insolublehydrophobic solvent. The organoamide may function to, for example,further bind to the REE, prevent formation of a third phase during theextraction, and/or assist in removing (stripping) the REE from theaqueous-insoluble hydrophobic solvent after extraction. To be soluble inthe hydrophobic solvent, the organoamide should be sufficientlyhydrophobic (lipophilic). To be sufficiently hydrophobic, theorganoamide should contain at least one hydrocarbon group containing atleast four carbon atoms. However, to ensure full solubility of theorganoamide in the hydrophobic solvent, the organoamide preferablycontains, in total, at least or more than six carbon atoms. In differentembodiments, the organoamide may contain at least or more than, forexample, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20carbon atoms, or a number of carbon atoms within a range bounded by anytwo of the foregoing values. Some examples of hydrophobic organoamidesinclude N-methylpentanamide, N-ethylpentanamide, N-propylpentanamide,N-butylpentanamide, N-pentylpentanamide, N-hexylpentanamide,N-methylhexanamide, N-ethylhexanamide, N-propylhexanamide,N-methyloctanamide, N-ethyloctanamide, N-propyloctanamide,N-methyldecanamide, N-ethyldecanamide, N-propyldecanamide,N,N-dimethylpentanamide, N,N-diethylpentanamide,N,N-dipropylpentanamide, N,N-dibutylpentanamide, N,N-dihexylpentanamide,and N,N-diethyloctanamide.

In some embodiments, the extractant solution, as described above,further includes an alcohol soluble in the aqueous-insoluble hydrophobicsolvent. The alcohol generally functions to prevent or lessen formationof a third phase during the extraction. To be soluble in the hydrophobicsolvent, the alcohol should be sufficiently hydrophobic (lipophilic). Tobe sufficiently hydrophobic, the alcohol typically contains at least ormore than six carbon atoms. In different embodiments, the alcoholcontains at least or more than, for example, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19, or 20 carbon atoms, or a number of carbon atomswithin a range bounded by any two of the foregoing values. Some examplesof lipophilic alcohols include n-hexyl alcohol, 4-methyl-1-pentanol,n-heptanol, n-octanol, 6-methyl-1-heptanol, 2-ethyl-1-hexanol,n-decanol, n-dodecanol, n-tridecanol, isotridecanol, n-tetradecanol, andn-hexadecanol.

In another aspect, the present disclosure is directed to a method forextracting one or more rare earth elements from an aqueous sourcesolution containing the one or more rare earth elements. The term “rareearth element” or “rare earth metal,” as used herein, refers to at leastthe lanthanide elements (elements having an atomic number of 57-71). Therare earth elements may or may not also include scandium (Sc) andyttrium (Y). The rare earth elements may or may not also include one ormore of the actinide elements (elements having an atomic number of90-103).

At least one lanthanide element is typically present in the aqueoussource solution. The one or more lanthanide elements present in theaqueous source solution may include one or more of the followingelements: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu). In the aqueous source solution, therare earth elements are present in ionic form (e.g., Nd⁺³) and salt form(e.g., Nd₂(SO₄)₃). The aqueous source solution may also contain at leastone (or one or more) of any of the actinide elements, such as uranium(U) and/or thorium (Th). In some embodiments, one or more of any of theforegoing rare earth elements are not present in the aqueous sourcesolution.

In a first step of the extraction process (i.e., step (i)), the aqueoussource solution is acidified with an inorganic acid (mineral acid) toresult in an acidified aqueous source solution containing the rare earthelements and containing the inorganic acid in a concentration of 1-12 M.In different embodiments, the inorganic acid concentration of theaqueous source solution is precisely or about, for example, 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, or 12 M, or an inorganic acid concentrationwithin a range bounded by any two of the foregoing values (e.g., 1-12 M,2-12 M, 2-8 M, 2-6 M, 3-12 M, 1-8 M, 2-8 M, 3-8 M, 1-5 M, 2-5 M, 3-5 M,or 1-3 M), wherein the term “about” may correspond to +50%, +20%, or+10% of any of the foregoing values. The inorganic acid may be, forexample, a hydrohalide (i.e., HX, wherein X is typically Cl, Br, or I),sulfuric acid (H₂SO₄), nitric acid (HNO₃), or phosphoric acid (H₃PO₄).In some embodiments, one or more of the foregoing inorganic acids isexcluded from the acidified aqueous source solution. In particularembodiments, the inorganic acid is hydrochloric acid or hydrobromicacid, which may or may not be combined with another inorganic acid.

In a second step of the extraction process (i.e., step (ii)), theacidified aqueous source solution from step (i) is contacted with theabove-described aqueous-insoluble hydrophobic extracting solutioncontaining a diglycolamide compound of Formula (1). The term “contacted”or “contacting,” as used herein in reference to contacting of theaqueous and organic phases, generally refers to an intimate mixing ofthe aqueous and organic phases so as to maximize extraction of the oneor more rare earth elements from the aqueous phase to the organic phase.Methods of intimately mixing liquids are well known in the art. Forexample, the aqueous and organic phases may be placed in a container andthe container agitated. In some embodiments, the liquids are intimatelymixed by subjecting them to vortex mixing. Following mixing, the twophases are generally separated by means well known in the art, such asby standing or centrifugation. The foregoing described process amountsto an efficient liquid-liquid extraction process whereby one or morerare earth elements in the aqueous source solution is/are extracted, insome cases selectively, into the aqueous-insoluble hydrophobic solvent(organic phase).

The extraction process is generally capable of achieving a distributioncoefficient (D), which may also herein be referred to as an extractionaffinity, of at least 1 for one or more the rare earth elements, whereinD is the concentration ratio of the rare earth element in the organicphase divided by its concentration in the aqueous phase. In someembodiments, a D value of greater than 1 is achieved, such as a D valueof at least or above 2, 5, 10, 20, 50, 100, 150, 200, 250, 500, or 1000.The selectivity of the process can be characterized by the separationfactor (SF), wherein SF is calculated as the ratio of D for twodifferent ions, such as any two of the ions disclosed above, such asselectivity of an earlier lanthanide (e.g., Nd) relative to one or morelater lanthanides (e.g., Tb), in which particular case SF=D_(Nd)/D_(Tb).Selectivity is generally evident in an SF value greater than 1. In someembodiments, an SF value of at least or greater than 2, 5, 10, 20, 50,100, 150, 200, 250, 500, or 1000 is achieved.

In some embodiments, the extraction step (step ii) extracts one or morerare earth elements to a greater degree (i.e., by a greater D value)than one or more other rare earth elements. By extracting one or moreelements to a greater degree than one or more elements, the extractionstep is exhibiting a degree in selectivity. The degree of selectivitycan be adjusted by, for example, selection of the extracting moleculeaccording to Formula (1); selection of the concentration of theextracting molecule in the hydrophobic solution; and selection of theinorganic acid and acid concentration in the aqueous source solution.For example, depending on the foregoing conditions employed, theextraction step may extract one or more early lanthanide elements (e.g.,La, Ce, Pr, and/or Nd) to a lesser or greater extent (i.e., at a lower Dvalue or higher D value, respectively) than one or more later lanthanideelements (e.g., Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and/or Lu) or Y. Asanother example, the extraction step may extract one or more lanthanideelements to a greater extent than one or more actinide elements (e.g.,Th and/or U), or vice-versa.

In some embodiments, the extraction method described above furtherincludes a successive stripping step (step (iii)). In the strippingstep, one or more rare earth elements contained in the aqueous-insolublehydrophobic solution is contacted with an aqueous stripping solutioncontaining at least one inorganic acid, such as any of the inorganicacids described above, wherein the inorganic acid is typically presentin a concentration of no more than 4 M. Generally, the concentration ofinorganic acid in the aqueous stripping solution is at least 0.5 M less(or at least 1 M, 1.5 M, 2 M, 3M, or 4M less) than the concentration ofinorganic acid in the aqueous source solution in step (i). In differentembodiments, inorganic acid concentration in the stripping solution isprecisely, about, up to (no more than), or less than, for example, 4 M,3.5 M, 3 M, 2.5 M, 2 M, 1.5 M, 1 M, 0.5 M, 0.25 M, 0.1 M, 0.05 M, 0.02M, or 0.01 M, or a concentration within a range bounded by any two ofthe foregoing values (e.g., 0.01-4 M, 0.01-3 M, 0.01-2 M, 0.01-1 M,0.01-0.5 M, 0.01-0.2 M, or 0.01-0.1M). Typically, the lower acidconcentration in the stripping solution favors removal (extraction) oflighter lanthanides over heavier lanthanides and actinides (from thehydrophobic solution into the stripping solution).

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

Examples

Diglycolamide (DGA) Based Compounds as Rare Earth Metal Extractants

In the following experiments, ligands are described that have a highaffinity for lanthanide elements, herein denoted as Ln(III) elements,that operate at higher ligand concentrations, such as 0.5 M, withoutforming a third-phase gel or precipitate and that operate at lower acidconcentrations, e.g., 1-3 M. The superior performance of these ligandsresults in significant cost savings. Moreover, these novel ligands canbe used in a solid phase for liquid-solid separation schemes, such aschromatographic separations.

The following extractant compounds were studied, while noting thatTODGA, TEHDGA, and DMDODGA are well known compounds that have beenincluded for comparative purposes:

Synthesis of Extractant Compounds

DGA ligands 4-17 were synthesized using the following syntheticprotocol:

The above scheme (a) shows synthesis of extractant compounds with linearand branched N-alkyl substituents. The above scheme (b) shows synthesisof extractant compounds containing pyrrolidine rings. In the aboveschemes, diglycolyl chloride was reacted with 1° or 2° amine to produceeither di- or tetra-substituted DGA, respectively. The di-substitutedDGAs were further functionalized using alkyl iodide as an alkylatingagent and sodium hydride as a base to produce tetra-substituted DGAs.Furthermore, a novel strategy was included that reduces steric crowdingaround the metal ion binding site by converting linear N,N′-alkylsubstituents into a five-membered pyrrolidine, which is represented byDGA ligand 11. Compound 11 was synthesized by free radical inducedaddition of alkylthiol (R′—SH) to N,N′-diallyl substituted DGA, as shownin scheme (b).

Detailed Synthesis for Compounds 4-8, 12, and 14-18

The following general synthetic scheme was used:

General procedure for the synthesis of N,N′-monosubstituted2,2′-oxybisacetamide ligands SI-1a-d: Compounds SI-1a-d were preparedfollowing this procedure. In a round-bottom flask equipped with a stirbar, an amine (2.1 equiv.) was combined with anhydrous CH₂Cl₂ (0.2 M)and Et₃N (2.1 equiv.). The reaction mixture was cooled in an ice-waterbath prior to slow addition of 2,2-oxydiacetyl chloride A (1 equiv.).The reaction mixture was allowed to warm up to room temperature and stirfor 1 hour. Afterwards, Et₂O (˜0.1 M) was added in one portion, theprecipitate was removed via filtration through a short Celite® plug andrinsed with Et₂O (3×) to yield crude product. See below for additionaldetails.

2,2′-oxybis(N-(2-ethylhexyl)acetamide) SI-1a was synthesized accordingto the general procedure using 2-ethylhexyl-1-amine (9.9 mL, 61 mmol) asan amine source. The crude product was purified on CombiFlash® R_(f)automated flash chromatography system using normal phase silica gel as astationary phase and gradient 0-40% MeOH in CH₂Cl₂ as an eluent system(R_(f)=0.8, 10% MeOH/CH₂Cl₂) to yield light brown oil (10.2 g, 99%). ¹HNMR (400 MHz, CDCl₃) δ 6.36 (br s, 2H), 4.04 (s, 4H), 3.28-3.24 (m, 4H),1.47-1.45 (m, 2H), 1.34-1.28 (m, 16H), 0.91-0.88 (m, 12H).

2,2′-oxybis(N-(5,9-dimethyldecyl)acetamide) SI-1b was synthesizedaccording to the general procedure using 5,9-dimethyldecan-1-amine (12.5mL, 54 mmol) as an amine source. The crude product was purified onCombiFlash® R_(f) automated flash chromatography system using normalphase silica gel as a stationary phase and gradient 0-40% MeOH in CH₂Cl₂as an eluent system (R_(f)=0.5, 5% MeOH/CH₂Cl₂) to yield yellow oil(12.0 g, 99%). ¹H NMR (400 MHz, CDCl3) δ 6.41 (br s, 2H), 4.04 (s, 4H),3.33-3.28 (m, 4H), 1.54-1.48 (m, 6H), 1.37-1.20 (m, 14H), 1.15-1.06 (m,8H), 0.87-0.83 (m, 18H). ¹³C NMR (100.66 MHz, CDCl3) 168.6, 71.3, 39.4,39.3, 37.4, 36.8, 32.8, 30.0, 28.1, 24.9, 24.5, 22.8, 22.7, 19.7. HRMSm/z: [M+H]⁺, calculated for C28H56N2O3, 469.4369; found 469.4378.

2,2′-oxybis(N-(4-butyldecyl)acetamide) SI-1c was synthesized accordingto the general procedure using 4-butyldecylamine (1.71 g, 8 mmol) as anamine source. The crude product was purified on CombiFlash® R_(f)automated flash chromatography system using normal phase silica gel as astationary phase and gradient 0-40% MeOH in CH₂Cl₂ as an eluent system(R_(f)=0.4, 5% MeOH/CH₂Cl₂) to yield transparent oil (1.98 g, 99%). ¹HNMR (400 MHz, CDCl₃) 6.40 (br s, 2H), 4.04 (s, 4H), 3.31-3.26 (m, 4H),1.53-1.49 (m, 2H), 1.25-1.23 (m, 40H), 0.90-0.87 (m, 12H). ¹³C NMR(100.66 MHz, CDCl₃) 168.4, 71.4, 39.7, 37.3, 33.7, 33.3, 32.1, 30.9,29.9, 29.0, 27.0, 26.8, 23.3, 22.9, 14.3, 14.3. HRMS m/z: [M+H]⁺,calculated for C32H64N2O3, 525.4995; found 525.5009.

2,2′-oxybis(N-octylacetamide) SI-1d was synthesized according to thegeneral procedure using octylamine (13.2 mL, 82 mmol) as an aminesource. After completion of the reaction, DI water (3×100 mL) was added,the organic layer was separated and washed with saturated NaCl solution,then dried over MgSO₄, filtered and concentrated under reduced pressure.Afterwards, Et₂O (100 mL) was added to the crude product and the formedcrystals were filtrated off. The product was obtained as a whitecrystalline powder (11.7 g, 84%). ¹H NMR (400 MHz, CDCl₃) 6.41 (br s,2H), 4.04 (s, 4H), 3.38-3.28 (m, 4H), 1.55-1.51 (m, 4H), 1.30-1.27 (m,20H), 0.88 (t, J=6.8 Hz, 6H).

General procedure for the synthesis of N,N′-disubstituted2,2′-oxybisacetamide ligands 4-8, 12, 14, 15: Compounds (ligands) 4-8,12, 14, 15 were prepared following this procedure. To a mixture of NaH(2.2 equiv.) in anhydrous THE (0.3 M) was added N,N′-monosubstituted2,2′-oxybisacetamide SI-1a-d dissolved in anhydrous THF (0.3 M) underinert atmosphere. The reaction mixture was stirred at room temperaturefor 15 minutes, before the addition alkyl iodide (3 equiv.) and furtherstirred at room temperature for ˜12 hours. To the reaction mixture EtOHwas added to neutralize excess NaH. The reaction mixture wasconcentrated under reduced pressure to yield crude product. See belowfor further details on purification.

2,2′-oxybis(N-(2-ethylhexyl)-N-methylacetamide) 4 was synthesizedaccording to the general procedure using2,2′-oxybis(N-(2-ethylhexyl)acetamide) SI-1a (4.35 g, 12 mmol) andmethyl iodide (2.24 mL, 36 mmol) as an alkyl iodide source. Product waspurified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 0-30%MeOH in CH₂C₂ as an eluent system (R_(f)=0.7, 10% MeOH/CH₂Cl₂) to yieldtransparent oil (4.21 g, 91%). ¹H NMR (400 MHz, CDCl₃) 4.32 (s, 4H),3.34-3.30 (m, 1H), 3.26-3.24 (m, 1H), 3.12-3.10 (m, 2H), 2.93-2.89 (m,6H), 1.64-1.58 (m, 2H), 1.26-1.22 (m, 16H), 0.88-0.84 (m, 12H). ¹³C NMR(100.66 MHz, CDCl₃) 169.4, 169.3, 169.1, 169.0, 69.6, 69.4, 69.1, 68.9,52.8, 51.3, 37.9, 37.0, 34.5, 33.5, 30.4, 30.4, 28.8, 28.7, 23.6, 23.1,23.0, 14.1, 14.1, 10.9, 10.6. HRMS m/z: [M+H]⁺, calculated forC22H44N2O3, 385.3430; found 385.3435.

2,2′-oxybis(N-(4-butyldecyl)-N-methylacetamide) 5 was synthesizedaccording to the general procedure using2,2′-oxybis(N-(4-butyldecyl)acetamide) SI-1c (2.03 g, 3.9 mmol) andmethyl iodide (0.75 mL, 12 mmol) as an alkyl iodide source. The productwas purified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 0-40%MeOH in CH₂Cl₂ as an eluent system (R_(f)=0.4, 5% MeOH/CH₂Cl₂) to yieldtransparent oil (1.9 g, 90%). ¹H NMR (400 MHz, CDCl₃) 4.32-4.30 (m, 4H),3.32 (t, J=7.4 Hz, 2H), 3.21 (t, J=7.5 Hz, 2H), 2.96 (s, 3H), 2.92 (s,3H), 1.53-1.48 (m, 2H), 1.24-1.22 (m, 40H), 0.90-0.86 (m, 12H). ¹³C NMR(100.66 MHz, CDCl₃) 169.0, 168.9, 168.8, 168.7, 69.7, 69.6, 69.3, 69.2,49.5, 48.4, 37.4, 33.7, 33.3, 32.1, 32.0, 29.9, 29.9, 29.02, 29.0, 26.8,26.7, 23.2, 22.8, 14.3, 14.3. HRMS m/z: [M+H]⁺, calculated forC34H68N2O3, 553.5308; found 553.5318.

2,2′-oxybis(N-(5,9-dimethyldecyl)-N-methylacetamide 6 was synthesizedaccording to the general procedure using2,2′-oxybis(N-(5,9-dimethyldecyl)acetamide) SI-1b (4.0 g, 9 mmol) andCH₃I (1.7 mL, 27 mmol) as an alkyl iodide source. The product waspurified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 0-30%MeOH in CH₂Cl₂ as an eluent system (R_(f)=0.7, 10% MeOH/CH₂Cl₂) to yieldtransparent oil (3.6 g, 81%). ¹H NMR (400 MHz, CDCl₃) 4.33-4.31 (m, 4H),3.36-3.32 (t, J=3.5 Hz, 2H), 3.24-3.20 (t, J=3.5, 2H), 2.96 (s, 3H),2.91 (s, 3H), 1.54-1.48 (m, 6H), 1.35-1.20 (m, 14H), 1.13-1.07 (m, 8H),0.87-0.82 (m, 18H). ¹³C NMR (100.66 MHz, CDCl₃) 169.0, 168.9, 169.8,168.7, 69.7, 69.6, 69.3, 69.2, 49.2, 48.2, 39.5, 37.4, 37.3, 37.0, 36.9,34.4, 33.4, 32.9, 28.8, 28.1, 27.6, 24.9, 24.5, 24.3, 22.9, 22.8, 19.8,19.7. HRMS m/z: [M+H]⁺, calculated for C30H60N2O3, 497.4682; found497.4691.

2,2′-oxybis(N-ethyl-N-octylacetamide) 7 was synthesized according to thegeneral procedure using 2,2′-oxybis(N-octylacetamide) SI-1d (4.35 g, 12mmol) and ethyl iodide (1.36 mL, 17 mmol) as an alkyl iodide source.Product was purified on CombiFlash® R_(f) automated flash chromatographysystem using normal phase silica gel as a stationary phase and gradient0-40% MeOH in CH₂Cl₂ as an eluent system (R_(f)=0.8, 60% EtOAc/Hexanes)to yield transparent oil (2.32 g, 85%). ¹H NMR (400 MHz, CDCl₃) 4.31 (s,4H), 3.39-3.34 (m, 2H), 3.29 (m, 4H), 3.20-3.16 (m, 2H), 1.52 (s, 4H),1.27 (s, 20H), 1.17-1.12 (m, 6H), 0.87 (s, 6H). ¹³C NMR (100.66 MHz,CDCl₃) 168.4, 69.5, 69.5, 69.3, 69.3, 46.8, 45.5, 45.5, 41.6, 40.7,31.9, 31.9, 29.5, 29.5, 29.4, 29.3, 22.8, 22.7, 14.3, 14.2, 13.0. HRMSm/z: [M+H]⁺, calculated for C24H48N2O3, 413.3743; found 413.3752.

2,2′-oxybis(N-octyl-N-propylacetamide) 8 was synthesized according tothe general procedure using 2,2′-oxybis(N-octylacetamide) SI-1d (2 g,5.6 mmol) and propyl iodide (1.67 mL, 17 mmol) as an alkyl iodidesource. Product was purified on CombiFlash® R_(f) automated flashchromatography system using normal phase silica gel as a stationaryphase and gradient 0-40% MeOH in CH₂C₂ as an eluent system (R_(f)=0.8,60% EtOAc/Hexanes) to yield transparent oil (2.35 g, 95%). ¹H NMR (400MHz, CDCl₃) 4.31 (s, 4H), 3.31-3.25 (m, 4H), 3.18-3.16 (m, 4H), 1.53 (m,8H), 1.26 (s, 20H), 0.88 (s, 12H). ¹³C NMR (100.66 MHz, CDCl₃) 168.8,168.8, 69.3, 69.2, 48.6, 47.5, 47.1, 45.9, 31.9, 31.9, 29.5, 29.5, 29.4,29.3, 27.7, 27.0, 22.8, 22.8, 22.2, 20.9, 14.2, 11.5, 11.3. HRMS m z.[M+H]⁺, calculated for C26H52N2O3, 441.4056; found 441.4067.

Detailed Synthesis for Compounds 9, 10, and 13

The following general synthetic scheme was used:

General procedure for the synthesis of N,N′-disubstituted2,2′-oxybisacetamide ligands 9-10: Compounds 9, 10 were preparedfollowing this procedure. To a mixture of R¹R²NH (1 equiv.) and R³R⁴NH(1 equiv.) in THE (0.3 M) was added Et₃N (2.1 equiv.). The reactionmixture was cooled in an ice-water bath prior to slow addition of2,2-oxydiacetyl chloride A (1 equiv.) then stirred at room temperaturefor 2 hours. The solvent was evaporated under reduced pressure. To thecrude material, Et₂O (˜0.3 M) was added, the precipitate was removed byfiltration through a short Celite® plug and rinsed with Et₂O (2×). Thefiltrate was concentrated under reduced pressure to yield crude product.See below for further details on purification.

2-(2-(dimethylamino)-2-oxoethoxy)-N,N-dioctylacetamide 9 was synthesizedaccording to the general procedure using 2 M dimethylamine in THE (6.0mL, 12 mmol) and dioctylamine (3.6 mL, 12 mmol) as secondary aminesources. The product was purified on CombiFlash® R_(f) automated flashchromatography system using normal phase silica gel as a stationaryphase and gradient 0-100% EtOAC in hexanes as an eluent system(R_(f)=0.4, 80% EtOAc/Hexanes) to yield transparent oil (1.68 g, 39%).¹H NMR (400 MHz, CDCl₃) 4.32 (s, 2H), 4.29 (s, 2H), 3.32-3.26 (m, 2H),3.20-3.13 (m, 2H), 3.01 (s, 3H), 2.95 (s, 3H), 1.52 (m, 2H), 1.26 (m,20H), 0.90-0.85 (m, 6H).

2-(2-(dioctylamino)-2-oxoethoxy)-N-methyl-N-octylacetamide 10 wassynthesized according to the general procedure usingN-methyl-N-octylamine (0.6 mL, 3.3 mmol) and dioctylamine (1.0 mL, 3.3mmol) as secondary amine sources. The product was purified onCombiFlash® R_(f) automated flash chromatography system using normalphase silica gel as a stationary phase and gradient 0-100% EtOAC inhexanes as an eluent system (R_(f)=0.2, 20% MeOH/CH₂Cl₂) to yieldtransparent oil (0.72 g, 45%). ¹H NMR (400 MHz, CDCl₃) 4.32-4.29 (m,4H), 3.34-3.26 (m, 6H), 2.96-2.91 (m, 3H), 1.52 (s, 6H), 1.27 (s, 30H),0.87 (br s, 9H). ¹³C NMR (100.66 MHz, CDCl₃) 169.0, 168.9, 168.8, 168.6,69.7, 69.3, 69.2, 49.2, 48.1, 47.1, 46.0, 46.0, 31.9, 31.9, 29.5, 29.5,29.5, 29.4, 29.4, 29.3, 27.2, 27.0.

Detailed Synthesis for Compound 11

The following general synthetic scheme was used:

2,2′-oxybis(N,N-divinylacetamide) B was synthesized according to thefollowing procedure. To a mixture of diallylamine (32.7 g, 128 mmol, 2.2equiv.) in THE (0.3 M) was added Et₃N (2.1 equiv.). The reaction mixturewas cooled in an ice-water bath prior to slow addition of2,2-oxydiacetyl chloride A (1 equiv.) then stirred at room temperaturefor 2 hours. The solvent was evaporated under reduced pressure. To thecrude material, Et₂O (˜0.3 M) was added, the precipitate was removed byfiltration through a short Celite® plug and rinsed with Et₂O (2×). Thefiltrate was concentrated under reduced pressure to yield crude product(23.6 g) that was used in the next step without further purification. ¹HNMR (400 MHz, CDCl₃) 5.80-5.70 (m, 4H), 5.21-5.12 (m, 8H), 4.30 (s, 4H),3.99 (d, J=6.0 Hz, 4H), 3.89 (d, J=5 Hz, 4H).

2,2′-oxybis(1-(3-methyl-4-(((2-methylundecan-2-yl)thio)methyl)pyrrolidin-1-yl)ethanone)11 was synthesized as follows: To a mixture of2,2′-oxybis(N,N-divinylacetamide) B (1 equiv.) and1,1-dimethyldecanethiol (3.0 equiv., 71 mL, 301 mmol) in MeOH (0.3 M)was added AIBN (15 mol %). The inert gas was bubbled through thereaction mixture for 30 minutes before heating it at 66° C. for 12hours. The solvent was evaporated to yield crude product. The productwas purified by column chromatography using normal phase silica gel as astationary phase and gradient 0-50% MeOH in CH₂Cl₂ as an eluent systemto yield orange oil (90.4 g, 95%). ¹H NMR (400 MHz, CDCl₃) 4.23-4.20 (m,4H), 3.89-3.78 (m, 1H), 3.70-3.65 (m, 1H), 3.52 (m, 2H), 3.33-3.04 (m,2H), 2.99-2.95 (m, 1H), 2.70-2.61 (m, 1H), 2.50-2.25 (m, 6H), 2.05-1.82(m, 4H), 1.58-1.06 (m, 28H), 0.93-0.80 (m, 26H). ¹¹C NMR (100.66 MHz,CDCl₃) 168.0, 167.9, 167.6, 167.4, 69.9, 69.8, 69.7, 69.5, 69.4, 69.3,53.5, 53.0, 52.8, 51.5, 51.0, 49.4, 49.0, 46.3, 42.4, 40.2, 39.1, 37.0,35.5, 33.4, 29.5, 29.1, 28.7, 25.8, 23.5, 22.8, 16.3, 14.7, 14.3, 13.3,13.1, 12.3, 11.6, 9.0, 8.8, 8.5. HRMS m/z: [M+H]⁺, calculated forC40H76N2O3S2, 697.5376; found 697.5380.

2,2′-oxybis(N-dodecyl-N-octylacetamide) 12 was synthesized according tothe general procedure using 2,2′-oxybis(N-octylacetamide) SI-1d (12.25g, 34 mmol) and dodecyl iodide (30.22 g, 102 mmol) as an alkyl iodidesource. The product was purified on CombiFlash® R_(f) automated flashchromatography system using normal phase silica gel as a stationaryphase and gradient 0-40% MeOH in CH₂Cl₂ as an eluent system (R_(f)=0.8,10% MeOH/CH₂Cl₂) to yield light brown oil (21.7 g, 92%). ¹H NMR (400MHz, CDCl₃) 4.33 (s, 4H), 3.31-3.27 (m, 4H), 3.19-3.15 (m, 4H),1.54-1.48 (m, 8H), 1.25 (m, 56H), 0.89-0.86 (m, 12H). ¹³C NMR (100.66MHz, CDCl₃) 168.8, 69.2, 47.1, 46.0, 32.1, 32.0, 31.9, 29.8, 29.7, 29.5,29.5, 29.5, 29.1, 27.7, 27.2, 27.0, 22.8, 22.8, 22.8, 14.3, 14.2. HRMSm/z: [M+Na]⁺, calculated for C44H88N2O3, 715.6687; found 715.6693.

Detailed Synthesis for Compound SI-3

The following general synthetic scheme was used:

N-(3,5,5-trimethylhexyl)octan-1-amine (SI-3). To a mixture of3,5,5-trimethylhexanal SI-2 (25 mL, 144 mmol) in MeOH (290 mL) was addedoctylamine (23.7 mL, 144 mmol). The reaction mixture was stirred at roomtemperature for 12 hours. Afterwards, NaBH₄ (6.5 g, 172 mmol) was addedin small portions and reaction mixture stirred at room temperature for 2hours. After the removal of reaction solvent on a rotary evaporator, 400mL of Et₂O were added. The suspension was filtrated through a shortsilica gel-Celite® plug and washed with Et₂O (2×). The filtrate wasconcentrated under reduced pressure to yield product (32.7 g, 89%) whichwas used in the next step without further purification. ¹H NMR (400 MHz,CDCl₃) 2.58 (t, J=7.3 Hz, 4H), 2.38 (m, 1H), 1.53-1.42 (m, 2H),1.35-1.20 (m, 14H), 1.07-1.03 (m, 1H), 0.93-0.86 (m, 15H).

2,2′-oxybis(N-octyl-N-(3,5,5-trimethylhexyl)acetamide) 13 wassynthesized according to the general procedure usingN-(3,5,5-trimethylhexyl)octan-1-amine 7 (21.8 mL, 176 mmol, 2.1 equiv.)as a secondary amine source. The product was purified on CombiFlash®R_(f) automated flash chromatography system using normal phase silicagel as a stationary phase and gradient 10-60% EtOAc in hexanes as aneluent system (R_(f)=0.6, 60% EtOAc/Hex) to yield transparent oil (25.0g, 71%). ¹H NMR (400 MHz, CDCl₃) 4.29 (s, 4H), 3.28-3.25 (m, 4H),3.17-3.14 (m, 4H), 1.52-1.49 (m, 24H), 1.09-1.05 (m, 2H), 0.95-0.86 (m,30H). ¹³C NMR (100.66 MHz, CDCl3) 168.5, 168.5, 69.2, 51.3, 51.2, 47.0,46.0, 45.5, 44.3, 38.5, 36.9, 31.9, 21.2, 20.1, 20.1, 29.4, 29.3, 27.7,22.8, 22.7, 14.2. HRMS m/z: [M+H]⁺, calculated for C38H77N2O3, 609.5934;found 609.5946.

2,2′-oxybis(N-(4-butyldecyl)-N-octylacetamide) 14 was synthesizedaccording to the general procedure using2,2′-oxybis(N-(4-butyldecyl)acetamide) SI-1c (19.03 g, 36 mmol) andoctyl iodide (19.5 mL, 108 mmol) as an alkyl iodide source. The productwas purified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 0-40%MeOH in CH₂C₂ as an eluent system (R_(f)=0.8, 10% MeOH/CH₂Cl₂) to yieldlight brown oil (25.6 g, 95%). ¹H NMR (400 MHz, CDCl₃) 4.37 (s, 4H),3.31-3.25 (m, 4H), 3.19-3.16 (m, 4H), 1.52-1.50 (m, 8H), 1.27-1.22 (m,58H), 0.90-0.86 (m, 18H). HRMS m/z: [M+H]⁺, calculated for C48H96N2O3,749.7494; found 749.7491.

2,2′-oxybis(N-(5,9-dimethyldecyl)-N-octylacetamide) 15 was synthesizedaccording to the general procedure using2,2′-oxybis(N-(5,9-dimethyldecyl)acetamide) SI-1b (20.17 g, 43 mmol) andoctyl iodide (23.3 mL, 129 mmol) as an alkyl iodide source. The productwas purified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 0-40%MeOH in CH₂C₂ as an eluent system (R_(f)=0.8, 10% MeOH/CH₂Cl₂) to yieldlight brown oil (28.6 g, 96%). ¹H NMR (400 MHz, CDCl₃) 4.32 (s, 4H),3.31-3.27 (m, 4H), 3.19-3.16 (m, 4H), 1.54-1.49 (m, 8H), 1.35-1.10 (m,44H), 0.87-0.82 (m, 24H). ¹³C NMR (100.66 MHz, CDCl₃) 168.9, 69.2, 47.2,46.1, 46.1, 39.5, 37.5, 37.4, 36.9, 36.9, 32.9, 31.9, 31.9, 29.5, 29.5,29.4, 29.0, 28.1, 28.0, 27.7, 27.2, 27.0, 24.9, 24.7, 24.5, 22.9, 22.8,22.8, 19.8, 19.7, 14.2. HRMS m/z: [M+H]⁺, calculated for C44H88N2O3,693.6868; found 693.6870.

Detailed Synthesis for Compound SI-5 and SI-6

The following general synthetic scheme was used:

N-hexyl-3, 7-dimethyloct-6-en-1-amine (SI-5) and N-(3,7-dimethyloct-6-en-1-yl)dodecan-1-amine (SI-6). To a mixture ofCitronellal SI-4 (90 mL, 0.5 mol) in MeOH (950 mL) was added eithern-hexylamine (66 mL, 0.5 mol) or n-dodecylamine (92.7 g, 0.5 mol). Thereaction mixture was stirred at room temperature for 12 hours.Afterwards, NaBH₄ (22.7 g, 0.6 mol) was added in small portions andreaction mixture stirred at room temperature for 2 hours. After theremoval of reaction solvent on a rotary evaporator, 400 mL of Et₂O wereadded. The suspension was filtrated through a short silica gel-Celite®plug and washed with Et₂O (2×). The filtrate was concentrated underreduced pressure to yield product which was used in the next stepwithout further purification.

2,2′-oxybis(N-(3, 7-dimethyloct-6-en-1-yl)-N-hexylacetamide) SI-7 and2,2′-oxybis(N-(3, 7-dimethyloct-6-en-1-yl)-N-dodecylacetamide) SI-8 weresynthesized according to the general procedure usingN-hexyl-3,7-dimethyloct-6-en-1-amine SI-5 (103 g, 0.43 mol, 2.1 equiv.)and N-(3,7-dimethyloct-6-en-1-yl)dodecan-1-amine SI-6 (10 g, 0.031 mol,2.1 equiv.) as a secondary amine source, respectively. Each product waspurified on CombiFlash® R_(f) automated flash chromatography systemusing normal phase silica gel as a stationary phase and gradient 10-60%EtOAc in hexanes as an eluent system (R_(f)=0.6, 60% EtOAc/Hex) to yieldtransparent oil (SI-7, 67.3 g, 60% and SI-8, 6.6 g, 57%).

2,2′-oxybis(N-(3, 7-dimethyloctyl)-N-hexylacetamide) 16 and2,2′-oxybis(N-(3, 7-dimethyloctyl)-N-dodecylacetamide) 17 weresynthesized via hydrogenation of 2,2′-oxybis(N-(3,7-dimethyloct-6-en-1-yl)-N-hexylacetamide) SI-7 and 2,2′-oxybis(N-(3,7-dimethyloct-6-en-1-yl)-N-dodecylacetamide) SI-8 using 10% Pd/C (2.4g/0.1 mol of SI-7 or SI-8) in MeOH (0.2 M) and H₂ (1 atm, balloon). Eachreaction mixture was filtrated through a short Celite® plug and washedwith MeOH. The filtrate was concentrated under reduced pressure to yieldproduct as light-yellow oil (99% yield). Compound 16: ¹H NMR (400 MHz,CDCl₃) 4.30 (s, 4H), 3.39-3.03 (m, 8H), 1.60-1.44 (m, 8H), 1.37-1.17 (m,22H), 1.17-1.04 (m, 6H), 0.95-0.75 (m, 24H). Compound 17: ¹H NMR (400MHz, CDCl₃) 4.29 (s, 4H), 3.39-3.09 (m, 8H), 1.59-1.45 (m, 8H),1.36-1.18 (m, 46H), 1.17-1.07 (m, 6H), 0.96-0.78 (m, 24H).

Detailed Synthesis for Compound 18

The following general synthetic scheme was used:

1-hexyl-3-((triisopropylsilyl)oxy)piperidin-2-one (SI-10).3-hydroxypiperidin-2-one SI-9 (2.5 g, 0.02 mol) and imidazole (1.05equiv) were dissolved in anhydrous DMF (0.2 M). To this solution wasthen added TIPS-Cl (1.05 equiv) and the reaction mixture was stirred atroom temperature for 12 hours. To the reaction mixture was added waterand product was extracted with E₂O (3×). The combined organic phase waswashed with brine, dried over MgSO₄, filtered, and solvent removed underreduced pressure. The product was used in the next step without furtherpurification. To the ice-cold solution of the TIPS protected product(5.9 g, 0.02 mol) in anhydrous THF (0.2 M) was added tBuOK (2.68 g, 0.02mol). The reaction mixture was stirred for 30 min before the addition ofn-hexyl iodide. Afterwards, the reaction mixture was stirred for 12hours at room temperature. To the reaction mixture was added water andproduct was extracted with E₂O (3×). The combined organic phase waswashed with brine, dried over MgSO₄, filtered, and solvent removed underreduced pressure. The product (SI-10) was used in the next step withoutfurther purification.

N,N-didodecyl-2-((1-hexyl-2-oxopiperidin-3-yl)oxy)acetamide (18). To1-hexyl-3-((triisopropylsilyl)oxy)piperidin-2-one SI-10 (0.02 mol)dissolved in anhydrous THE (0.4 M) was added TBAF (1 M in THF, 1.2equiv). The reaction mixture was stirred at room temperature for 12hours. Afterwards, the solvent was removed under reduced pressure andproduct was purified on CombiFlash® R_(f) automated flash chromatographysystem using normal phase silica gel as a stationary phase and gradient0-80% EtOAc in hexanes as an eluent system to yield light yellow oil(3.9 g, 90%). Next, the round bottom flask was charged with NaH (0.25 g,6.3 mmol) and anhydrous THE (0.2 M) under inert atmosphere. To thereaction mixture was then added dropwise the above obtained product(1.26 g, 6.3 mmol) dissolved in 5 mL of anhydrous THF. The reactionmixture was stirred at room temperature for 30 minutes. Afterwards,2-chloro-N,N-didodecylacetamide SI-11 (3.0 g, 6.3 mmol) dissolved inanhydrous THE (5 mL) was added to the reaction mixture. The reactionmixture was heated at 35° C. for 12 hours. To the reaction mixture wasadded water and product was extracted with E₂O (3×). The combinedorganic phase was washed with brine, dried over MgSO₄, filtered, andsolvent removed under reduced pressure. and product was purified onCombiFlash® R_(f) automated flash chromatography system using normalphase silica gel as a stationary phase and gradient 0-80% EtOAc inhexanes as an eluent system to yield light yellow oil (2.7 g, 73%). ¹HNMR (400 MHz, CDCl₃) 4.73-4.55 (m, 2H), 4.00-3.92 (m, 1H), 3.38-3.05 (m,8H), 2.24-2.12 (m, 1H), 2.07-1.94 (m, 2H), 1.86-1.65 (m, 3H), 1.60-1.45(m, 6H), 1.35-1.18 (m, 40H), 0.93-0.83 (m, 9H).

Detailed Synthesis for Compound 20

The following general synthetic scheme was used:

2-(2-(dioctylamino)-2-oxoethoxy)-N,N-dioctylbutanamide (20). To thesolution of TODGA (5.8 g, 10.0 mmol) in anhydrous THE (100 mL) at −90°C. was added dropwise LDA (2 M, 10.5 mmol). The reaction mixture wasallowed to warm up to −30° C. and at this temperature ethyl iodide (3.1g, 20 mmol) was added. The reaction mixture was stirred at roomtemperature for 12 hours. Afterwards, the solvent was removed underreduced pressure and product was purified on CombiFlash® R_(f) automatedflash chromatography system using normal phase silica gel as astationary phase and gradient 0-80% EtOAc in hexanes as an eluent systemto yield light yellow oil (3.6 g, 59%). ¹H NMR (400 MHz, CDCl₃)4.36-4.31 (m, 1H), 4.09 (dd, J=167.6, 13.8 Hz, 2H), 3.50-3.38 (m, 1H),3.38-3.13 (m, 7H), 1.86-1.66 (m, 2H), 1.58-1.44 (m, 8H), 1.36-1.18 (m40H), 1.01 (t, J=7.5 Hz, 3H), 0.94-0.82 (m, 12H).

Metal Extraction Experiments

General procedure: A 750 microliter (L) aqueous phase consisting of 7 mMLn(III) (0.5 mM of each Ln(III)) in 3 M HCl was contacted with an equalvolume of organic phase containing 0.1 M DGA ligands 1-13 in organicsolvents that was pre-equilibrated with the 3M HCl. The two phases werecontacted at a 1:1 ratio of organic/aqueous by end-over-end rotation inindividual 1.8 mL capacity snap-top Eppendorf tubes using a rotatingwheel in an air box set at 25.5° C.±0.5° C. Contacts were performed intriplicate with a contact time of 1 hour. Following contacting, thetriplicate samples were subjected to centrifugation at 1,811×g for twominutes at 20° C. to separate the phases. Each triplicate was thensubsampled, with 500 μL aliquots of the aqueous phases transferred toindividual polypropylene tubes containing 2.5 mL of 4% HNO3 for analysisusing ICP-OES. Two samples of the initial lanthanide solution were alsoprepared for the analysis, 500 μL were transferred to individualpolypropylene tubes containing 2.5 mL of 4% HNO₃. The areas found underthe observed peaks were used for determining distribution (D) values.

D Values are calculated according to the following equation:

D=[M]_(org)/[M]_(aq)  Eq. 1

Separation factors are calculated according to the following equation:

SF=D _(Ln1) /D _(Ln2) =X/Y  Eq. 2

The separation of trivalent lanthanides from hydrochloric acid in thisstudy was performed under conditions that are relevant to REE refining.Readily available isoparaffinic hydrocarbon (Isopar L) mixed with 30 vol% of branched aliphatic alcohol (Exxal 13) was used as an oil phase and3 M hydrochloric acid in water as an aqueous phase. The use of aliphaticalcohol (30 vol %) as a phase modifier prevents third-phase or gelformation. The distribution ratios (D), calculated from competitivemetal ion extraction studies, where the concentration of metal ion inthe organic phase is divided by its concentration in the aqueous phase,simultaneously provide information on the extraction strength,selectivity, and the point in the series at which these effects are nolonger noticeable (the leveling effect). The selectivity of separatingtwo metal ions is expressed as the separation factor, defined as theratio of one metal to that of another; for example,SF_(Lu/La)=D_(Lu)/D_(La). All experiments were performed in triplicateusing ligand 1 as a control for all measurements presented in this work.

Prior to extraction, the prepared organic phases were pre-equilibratedwith 3 M HCl solution, i.e., identical solutions as used in extraction,only in the absence of the studied metal. For that, one volume of theorganic phase was agitated for 10 minutes with three-fold volume of 3 MHCl, and agitation was repeated two more times. The equilibrated organicphases were contacted with aqueous phases containing 1.2 mmol/L of thestable Eu and activity concentration 3.7E+4 Bq/mL of Eu in 1:1 o:avolume ratio in 1.8 mL capacity snap-top Eppendorf tubes. Extraction wasperformed via vigorous shaking for 60 minutes at 4500 rpm using a vortexmixer at 25° C., followed by centrifugation at 10000 rpm for 5 minutes.All extractions were performed in duplicate, and values of distributionratios were determined using Eq.1, as a ratio of activity concentrationsin organic and aqueous phases.

Phases were separated using a fine tipped transfer pipet and each samplewas then sub-sampled, with 300 μL aliquots of the organic and aqueousphases transferred to individual polypropylene tubes for gamma counting.The activity concentrations were measured using gamma spectroscopy of300 μL aliquots of each liquid phase, using a NaI(Tl) scintillationcounter (Packard COBRA II). The peaks in gamma-spectrum considered werethose between 105-143 keV. The measurement times were set in order toobtain a relative standard deviation of the counting statistics lowerthan 1%.

FIG. 1 is a graph plotting variation of log D in the extraction ofLn(III), 0.5 mM each, with 0.1 M DGA ligands 1 (TODGA), 2 (TEHDGA), and3 (DMDODGA) from 3 M HCl media into Isopar L with 30 vol % of Exxal 13at 25° C. after 1 hour. The dotted horizontal line represents theleveling off point. The nonlinear ascending trend is observed across theLn(III) series by diglycolamide ligands and may be explained by theincreased ion-dipole interactions between the extractant molecules andLn(III) with smaller ionic radius and the ability of extractantmolecules to dehydrate Ln(III) during the separation process. It isknown that the hydration energy of Ln(III) ions increases and ionicradius decreases with increasing atomic number. Since the electrondensity of the oxygen atoms for the DGA ligands is comparable (withexception of ligand 11), the change in Log D versus Ln(III) may beattributed to the conformational changes in DGA molecules due to thepresence of N,N′-alkyl substituents of varying size. A minimization ofallylic strain (A1,3 strain) between the alkyl substituents on amidenitrogen and amide carbonyl group as well as minimization of eclipsinginteractions in alkyl chains is needed to eliminate disfavoringinteractions. These disfavoring interactions increase as the alkylsubstituents get longer and larger in size and affect the accessibilityof Ln(III) ions to complex with oxygen donor atoms in DGA.

The reduction of the steric hinderance around the metal center(DMDODGA<TODGA<TEHDGA) results in a dramatic increase in extractionstrength (FIG. 1). The replacement of N,N-n-octyl substituents in 1 withmethyl groups (3) results in a large increase in D ratios for allLn(III), but lower overall selectivity, since even the most poorlyextracted La(III) has an increase in D ratio by over an order ofmagnitude (Table 1 below).

TABLE 1 Dependence of alkyl substituents length, size and branchingpoint on the separation of Ln(III) from 3M HCl using 0.1M Ligand inIsopar L with 30% v/v Exxal 13 at 25° C. BDL: below detection limit.Branch- Ligand R R′ ing DLa D_(Sm) SF_(Sm/La) D_(Pr) D_(Nd) SF_(Nd/Pr) 1 (TODGA) n-octyl n-octyl — 0.08 ± 0.47 5.99 ± 0.07 75 0.34 ± 0.15 0.85± 0.09 2.5  2 (TEHDGA) 2-ethylhexyl 2-ethylhexyl β 0.04 ± 0.01 0.11 ±0.01 3 0.07 ± 0.01 0.08 ± 0.01 1.1  3 (DMDODGA) methyl n-octyl — 4.15 ±0.02 259.09 ± 0.17  62 17.45 ± 0.01  48.22 ± 0.05  2.8  4 methyl2-ethylhexyl β 0.78 ± 0.04 40.89 ± 0.29  52 4.64 ± 0.06 11.65 ± 0.12 2.5  5 methyl 4-butyldecyl δ 0.14 ± 0.06 2.66 ± 0.05 19 0.34 ± 0.04 0.60± 0.04 1.8  6 methyl 5,9-dimethyldecyl ε 0.66 ± 0.04 8.37 ± 0.20 13 1.50± 0.06 2.66 ± 0.08 1.8  7 ethyl n-octyl — 0.73 ± 0.04 61.94 ± 0.10  854.89 ± 0.05 10.35 ± 0.05  2.1  8 propyl n-octyl — 0.34 ± 0.07 38.47 ±0.10  113 2.84 ± 0.05 5.86 ± 0.05 2.1  9 N,N′-di(n- N,N′-dimethyl — 3.69± 0.04 BDL — 22.30 ± 0.08  70.54 ± 0.16  3.2 octyl) 10 N,N′-di(n-N-methyl-N′-n- — 0.34 ± 0.10 23.16 ± 0.08  68 2.01 ± 0.07 3.61 ± 0.071.8 octyl) octyl 11 — 2-methyldecan-2-yl — 1.93 ± 0.07 21.49 ± 0.05  114.65 ± 0.05 5.56 ± 0.06 1.2 12 n-octyl n-decyl — 0.08 ± 0.07 11.14 ±0.04  145 0.53 ± 0.03 1.44 ± 0.05 2.7 13 n-octyl 3,5,5-trimethylhexyl γ0.05 ± 0.07 1.52 ± 0.02 30 0.37 ± 0 02 0.28 ± 0.03 0.8 14 n-octyl4-butyldecyl δ 0.00 ± 0.05 0.67 ± 0.09 141 0.03 ± 0.03 0.10 ± 0.04 3.115 n-octyl 5,9-dimethyldecyl ε 0.04 ± 0.03 4.21 ± 0.04 97 0.23 ± 0.020.58 ± 0.05 2.5

Notably, nearly quantitative (>97%) extraction of lanthanides is reachedstarting at Nd(III) using compound 3 (ligand 3) and at Tb(III) whenusing compound 1 (ligand 1). In the separation systems involving all 15lanthanides it has herein been shown that the elements may be groupedinto four tetrads using a variety of ligands. A similar effect in theordering of Ln(III) can be seen in FIG. 1 for ligands 1 and 3 throughthe use of three curved lines for La—Ho series; the effect after Ho(III)is somewhat unclear. Ligand 2 is by far the weakest DGA ligand in theseries, even though the inductive effects should favor stronginteractions with metal ions, the steric hindrance originated by ethylsubstituents at the p position on N,N,N′,N′-hexyl substituentseffectively shield the metal ion binding site. The replacement of twoout of four 2-ethylhexyl substituents in 2 with methyl groups has moreconsiderable effect on ligand 4 performance than does the change ofn-octyl substituents in 1 with two methyl groups (ligand 3).

As shown in FIG. 2A, overall, ligand 4 surpasses ligand 1 in terms ofextraction strength and shows an increase in D ratio by over two ordersof magnitude in comparison to 2. As shown in FIG. 2B, the effects ofmoving the branching point further away from the binding site in DGAligand (from β site in 4 to 6 in 5, to e carbon in 6) are pronounced interms of the extraction strength and selectivity across the Ln(III)series. Ligand 4, which has nearly two times fewer carbons than ligands5 and 6, performs better in terms of an extraction strength (see FIG.3B). Ligands 3 and 4 have the same overall carbon count, but due toproximity of the branching sites in ligand 4 to the binding site, theextraction efficiency drops on average 1.3 times (see FIG. 4B). The morestructurally similar ligands 5 and 6 show comparable extraction strengthof Ln(III), however, their selectivity across the Ln(III) series isnotably different. Similar to ligand 4, the extraction curve of ligand 5levels off starting at Tb(III), whereas ligand 6 shows a steady increasein extraction throughout the series with good selectivity among adjacentlanthanides (e.g., SF_(Eu/Sm)=2.2, SF_(Tb/Gd)=1.9, SF_(Er/Ho)=2.0). Atrend in the series of ligands (e.g., for 3, 7, and 8) is that theextraction strength increases with shortening of the alkyl substituent(FIGS. 2C and 3A). Interestingly, this effect is only pronounced in theearly lanthanide region (from La to Sm), since all three ligands showcomparable D ratios for Eu through Lu.

The structural isomer of 3, ligand 9, shows different interfacialproperties. Among all ligands, only ligand 9 displayed the third phaseformation at 0.1 M ligand concentration when contacted with 3 M HClsolution containing Ln(III). The analysis of aqueous phase afterextraction shows steady increase in D ratios from La(III) to Tb(III) anda complete absence of heavier elements (Dy, Er—Lu), which either areheld by the ligand in the oil phase or reside as a white precipitate atthe oil and water interface. Among all DGA ligands tested, ligand 9showed the greatest extraction strength, consistent with markedlyreduced steric hindrance around the 3-oxygen donor binding site (FIG.4A). Compound 10, an intermediate ligand between 1, 3 and 9 with onlyone N-methyl substituent, performs as expected, extracts less of Ln(III)than ligands 3 and 9 but is a stronger extractant than ligand 1 (FIGS.2D and 4A).

The ligand 11 provides an alternative strategy for reduction of sterichindrance around the metal ion binding site, while maintaining goodinterfacial properties through the inclusion of long, branched alkylsubstituents on pyrrolidines. Based on the results displayed in FIG. 2Eand Table 1, the conversion of linear N,N′-alkyl substituents into a5-membered N-heterocycle results in markedly improved extractionstrength of Ln(III). For instance, ligand 11 is a stronger extractantthan ligand 1 but displays similar selectivity trend across the Ln(III)series to that of ligand 3. The lower extraction strength of ligand 11relative to ligand 3 can be attributed to a presence of longer alkylsubstituents, which, similarly to ligands 5 and 6 in respect to ligand 3(FIG. 4B), results in suppressed extraction power of DGA ligands. Areverse takes place when the linear N-alkyl substituents are extendedbeyond eight carbons, with ligand 12 in FIG. 2F serving as an example.

The extension of two N-alkyl groups in DGA by four carbons results inimproved extraction of all Ln(III), which is counterintuitive to whathas been presented above. There may be a change in performance whenconsidering DGA ligands with linear N-alkyl substituents, likely TODGA(1) with four N-octyl substituents representing the turning point (FIG.3A). While it is true that DGAs with shorter alkyl substituents (lessthen C8) are generally stronger extractants of Ln(III) then TODGA (1),mainly due to reduced steric hindrance around the metal binding site, adifferent mechanism appears to take place when the N-alkyl substituentlength is increased beyond eight carbons, reversing the extractiontrend.

Notably, the N,N-di(2-ethylhexyl)-N′,N′-dioctyldiglycolamide (DEHDODGA)in comparison to ligand 2 was shown to be ineffective in improving theextraction efficiency of Ln(III) from 1.5 M nitric acid (D_(La)=0.52 andD_(Lu), =3.40 vs. D_(La)=0.72 and D_(Lu)=6.35 using ligand 2). Theplacement of the branching point on an N-alkyl substituent further awayfrom the metal ion binding site (from β to γ, δ, or ε position) has aprofound positive effect on DGA's performance and results in improved Dratios and selectivity profile across the Ln(III) series relative toligand 2 (FIG. 3C).

Even though ligands 13, 14, and 15 are not isostructural, due toinaccessibility of such branched 1° amines, halides, aldehydes, oralcohols required for the synthesis, they show a notable change inseparation of Ln(III). As shown in FIGS. 2G and 4C, the ligands 13 and14 with branching at γ and δ sites, respectively, are weaker extractantsthan ligands 1 and 12, likely due to closer proximity of these tertiarycarbon sites to the metal ion binding site. When the tertiary carbonsite is farthest from the amide nitrogen atom, as in ligand 15,extraction strength and selectivity increase and resemble that ofligand 1. Ligands 12 and 15 are isostructural, each have two n-octyl andtwo C₁₂H₂₅ N-substituents, and even though their performance iscomparable (FIG. 4C and Table 1), the less bulky ligand 12 extractsLn(III) more efficiently. This ligand-pair, in particular, demonstratesthat subtle changes to the ligand's structure have a significant impacton its performance in Ln(III) separation.

FIG. 5 plots data useful for determining the stoichiometry of extractedspecies for new lipophilic ligands 11 and 13. The results suggest thatligand 13 similarly to ligand 1 forms 3:1 ligand to Eu(III) complexes inthe organic phase, whereas 2:1 species are formed with ligands 2 and 11.The result for ligand 11 is unexpected since it is a substrate withsubstantially reduced steric hindrance around the three O-donor bindingsite, similar to ligand 3.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A rare earth extractant compound having thefollowing structure:

wherein: R¹, R², R³, and R⁴ are independently selected from alkyl groupscontaining 1-30 carbon atoms and optionally containing an ether orthioether linkage connecting between carbon atoms, provided that thetotal carbon atoms in R¹, R², R³, and R⁴ is at least 12; and R⁵ and R⁶are independently selected from hydrogen atom and alkyl groupscontaining 1-3 carbon atoms; and provided that at least one of thefollowing conditions apply: (i) at least one of R¹, R², R³, and R⁴ is adistal branched alkyl group constructed of a linear alkyl backbonehaving at least four carbon atoms with an alpha carbon atom of thelinear alkyl backbone attached to a nitrogen atom shown in Formula (1),and the linear alkyl backbone contains a substituting hydrocarbon groupat a gamma carbon or higher positioned carbon on the linear alkylbackbone, wherein the substituting hydrocarbon group contains at leastone carbon atom, provided that the distal branched alkyl group containsa total of up to 30 carbon atoms; (ii) R¹ and R² are equivalent and R³and R⁴ are separately equivalent, while R¹ and R² are different from R³and R⁴, or alternatively, only one of R¹, R², R³, and R⁴ is different,to result in an asymmetrical compound of Formula (1); (iii) R¹ and R²interconnect to form a first amine-containing ring, and/or R³ and R⁴interconnect to form a second amine-containing ring, and the first andsecond amine-containing rings are attached to an alkyl group containingat least three carbon atoms and optionally containing an ether orthioether linkage connecting between carbon atoms; and/or (iv) R² and R⁵interconnect to form a first lactam ring, and/or R⁴ and R⁶ interconnectto form a second lactam ring.
 2. The compound of claim 1, whereincondition (i) applies and wherein the distal branched alkyl groupcontains at least one substituting hydrocarbon group located at a gammacarbon of the linear alkyl backbone.
 3. The compound of claim 1, whereincondition (i) applies and wherein the distal branched alkyl groupcontains at least one substituting hydrocarbon group located at a deltacarbon of the linear alkyl backbone.
 4. The compound of claim 1, whereincondition (i) applies and wherein the distal branched alkyl groupcontains at least one substituting hydrocarbon group located at anepsilon carbon of the linear alkyl backbone.
 5. The compound of claim 1,wherein condition (i) applies and wherein the distal branched alkylgroup contains at least two substituting hydrocarbon groups located atgamma or higher carbon positions of the linear alkyl backbone.
 6. Thecompound of claim 1, wherein condition (ii) applies and wherein R¹ andR² are equivalent and each contains 1-3 carbon atoms, and R³ and R⁴ areseparately equivalent and each contains 4-30 carbon atoms.
 7. Thecompound of claim 1, wherein condition (ii) applies and wherein R¹ andR² are equivalent and each contains 1-3 carbon atoms, and R³ and R⁴ areseparately equivalent and each contains 6-30 carbon atoms.
 8. Thecompound of claim 1, wherein condition (ii) applies and wherein R¹ andR² are equivalent and each contains 1-3 carbon atoms, and R³ and R⁴ areseparately equivalent and each contains 8-30 carbon atoms.
 9. Thecompound of claim 1, wherein condition (iii) applies and wherein saidfirst or second amine-containing ring is a five-membered ring.
 10. Thecompound of claim 1, wherein condition (iii) applies and wherein saidalkyl group contains at least three carbon atoms and an ether orthioether linkage connecting between carbon atoms.
 11. The compound ofclaim 1, wherein condition (iii) applies and wherein said alkyl groupcontains at least six carbon atoms and an ether or thioether linkageconnecting between carbon atoms.
 12. The compound of claim 1, whereincondition (iv) applies.
 13. A liquid solution useful for extracting rareearth elements from aqueous solutions, the solution comprising a rareearth extractant compound dissolved in an aqueous-insoluble hydrophobicsolvent, wherein the rare earth extractant compound has the followingstructure:

wherein: R¹, R², R³, and R⁴ are independently selected from alkyl groupscontaining 1-30 carbon atoms and optionally containing an ether orthioether linkage connecting between carbon atoms, provided that thetotal carbon atoms in R¹, R², R³, and R⁴ is at least 12; and R⁵ and R⁶are independently selected from hydrogen atom and alkyl groupscontaining 1-3 carbon atoms; and provided that at least one of thefollowing conditions apply: (i) at least one of R¹, R², R³, and R⁴ is adistal branched alkyl group constructed of a linear alkyl backbonehaving at least four carbon atoms with an alpha carbon atom of thelinear alkyl backbone attached to a nitrogen atom shown in Formula (1),and the linear alkyl backbone contains a substituting hydrocarbon groupat a gamma carbon or higher positioned carbon on the linear alkylbackbone, wherein the substituting hydrocarbon group contains at leastone carbon atom, provided that the distal branched alkyl group containsa total of up to 30 carbon atoms; (ii) R¹ and R² are equivalent and R³and R⁴ are separately equivalent, while R¹ and R² are different from R³and R⁴, or alternatively, only one of R¹, R², R³, and R⁴ is different,to result in an asymmetrical compound of Formula (1); (iii) R¹ and R²interconnect to form a first amine-containing ring, and/or R³ and R⁴interconnect to form a second amine-containing ring, and the first andsecond amine-containing rings are attached to an alkyl group containingat least three carbon atoms and optionally containing an ether orthioether linkage connecting between carbon atoms, wherein the totalnumber of carbon atoms in the first or second amine-containing ring andattached alkyl group is up to 30 carbon atoms; and/or (iv) R² and R⁵interconnect to form a first lactam ring, and/or R⁴ and R⁶ interconnectto form a second lactam ring.
 14. The liquid solution of claim 13,wherein the aqueous-insoluble hydrophobic solvent is a hydrocarbonsolvent.
 15. The liquid solution of claim 13, wherein the liquidsolution further comprises an organoamine soluble in theaqueous-insoluble hydrophobic solvent.
 16. The liquid solution of claim15, wherein the organoamine contains at least one hydrocarbon groupcontaining at least four carbon atoms.
 17. The liquid solution of claim13, wherein the liquid solution further comprises an alcohol soluble inthe aqueous-insoluble hydrophobic solvent, and wherein the alcoholcontains at least six carbon atoms.